Cosmic gamma ray bursts
(GRBs) were discovered by accident in the late 1960's by satellites designed to
detect gamma rays produced
by atomic bomb tests on Earth. The GRBs appear first as a brilliant flash of
gamma rays, that rises and falls in a matter of minutes. These bursts are often
followed by afterglows at X-ray, optical and radio wavelengths.

A major leap forward in understanding the source of cosmic GRBs was made when
the Burst and Transient Source Experiment (BATSE) was launched aboard the Compton
Gamma Ray Observatory in 1991.

BATSE had an all-sky monitor that was capable of detecting a GRB virtually
anywhere in the sky. Over a period of 9 years BATSE recorded thousands of GRBs,
about 1 per day. Among other things, these results showed that the bursts
occurred at random all over the sky. If the bursts were associated with objects
in our Milky Way
Galaxy, they would not show such a universal distribution. Rather, they would
be concentrated along the plane of our galaxy like most of the matter in the
Milky Way. The BATSE data was so good that it allowed astronomers to also rule
out the possibility that the GRBs might be originating in the halo of our
galaxy.

2704 BATSE Gamma Ray Burst (Credit: NASA)

Beppo-Sax Satellite

Artist's concept of
a gamma-ray burst
(Credit: STScI)

In 1997, astronomers were able to use the Beppo-Sax satellite to refine the
location of several GRBs by observing their X-ray afterglow. Then the Hubble Space Telescope and other optical telescopes were
used to study the optical afterglow of the GRBs and were able to precisely locate
them in galaxies billions of light years from Earth. At such great distances, a
GRB must produce enormous amounts of energy. While at their peak, which lasts
only a few seconds, they have a power output that is comparable to that of all
the galaxies in the universe!

The source of this tremendous energy is unknown. Astronomers have developed a
model – the fireball model – that explains the time variation of the
bursts, and the shift of the peak radiation to progressively lower energies
reasonably well. The model involves matter moving at near the speed of light that
collides with other material in the vicinity.

What is the source of this rapidly moving matter? Theories include the merging of
neutron stars, or
black holes, or the
collapse of an extremely massive star to produce what has been called a
hypernova.

In one variation of this model, the collapsed core forms a spinning black hole.
As surrounding material falls toward this black hole, intense beams of high
energy particles and neutrinos eject matter at nearly the speed of light. It is
this matter that produces the gamma ray fireball.

X-ray observatories such as Chandra should help to solve the mystery of gamma ray
bursts. By studying the X-ray afterglow, they can measure the amount of gas in
the vicinity of the burst, and tell which elements are present. This should help
to pin down which theory is correct. For example, the Chandra observation of GRB991216 provides evidence for a large,
iron-rich cloud moving away from the site of the burst. Although much more work
needs to be done, this observation would appear to support the hypernova
model.